Typically, a polyimide (PI) film is formed from polyimide, and polyimide refers to a highly heat-resistant resin prepared by subjecting an aromatic dianhydride and an aromatic diamine or an aromatic diisocyanate to solution polymerization, thus preparing a polyamic acid derivative, which is then subjected to ring-closing reaction and dehydration at high temperature so as to be imidized.
Useful in the preparation of such polyimide, the aromatic dianhydride may include pyromellitic dianhydride (PMDA), biphenyltetracarboxylic dianhydride (BPDA), etc., and the aromatic diamine may include oxydianiline (ODA), p-phenylenediamine (p-PDA), m-phenylenediamine (m-PDA), methylenedianiline (MDA), bis aminophenyl hexafluoropropane (HFDA), etc.
Polyimide is a very highly heat-resistant resin which is nonsoluble and nonfusible, and is superior in terms of thermal oxidation resistance, heat resistance, radiation resistance, low-temperature characteristics, chemical resistance, etc., and has thus been utilized in a variety of fields including advanced heat-resistant materials, such as automotive materials, aircraft materials, spacecraft materials, etc., and electronic materials such as insulation coating materials, insulating films, semiconductors, electrode protecting films for TFT-LCDs, etc.
However, polyimide is brown- or yellow-colored due to its high aromatic ring density, and thus has low transmittance in the visible light range, making it difficult to apply polyimide to fields requiring transparency.
Hence, attempts have been made to impart transparency to polyimide which typically shows a deep brown or yellow color. For example, a linkage group (—O—, —SO2—, —CO—, —CF3CCF3—, etc.) or a side chain having a comparatively large free volume is introduced to a main chain, so that an intermolecular or intramolecular charge transfer complex is minimized to achieve transparency.
In the case of such a transparent polyimide film, the color based on the charge transfer complex fades away but heat resistance may be decreased due to the functional group introduced as above. Accordingly, polyimide having low heat resistance is difficult to apply to advanced material fields such as displays or semiconductors requiring high process temperatures.
Conventional polyimide, which has a high coefficient of thermal expansion, may easily incur warpage or entanglement when being used in advanced material fields such as displays or semiconductors and in optical coating films. Hence, a polyimide film is required to have a low coefficient of thermal expansion.
A filler, which is responsible for a variety of functions in the film, is used to increase traveling performance upon production of the film, or to enhance optical properties or heat resistance as needed.
The filler may be physically dispersed using a mill, a mixer, a high-speed stirrer-homogenizer, an ultrasonic processor, etc., and also, may be dispersed after surface treatment to prevent agglomeration of the dispersed filler and to increase miscibility.